In cultivation of melon plants, a disease caused by powdery mildew fungus is a serious problem worldwide. Plant bodies infected with the powdery mildew fungus grow poorly owing to withering of leaves, etc. As a result, the yield of fruits is reduced.
Under these circumstances, attempts have been made to breed melon cultivars resistant to the powdery mildew fungus utilizing powdery mildew resistance genes. However, appearance of species of powdery mildew fungi capable of infecting melon plants including these resistance genes gives rise to a problem (Non-Patent Document 1).